A conventional automated welding system using a robot, etc., is configured so that it measures a welding-groove position of work pieces using a sensor that projects a slit laser or scans with a spot laser, etc., before performing welding, and calculates (or corrects) an instruction value for the welding robot based on the measurement, in order to compensate for individual specificity and the misalignment of the work pieces.
For example, Japanese Patent No. 3228777 discloses an apparatus that includes a light source which projects a slit laser light on work pieces with the line of sight being perpendicular to a welding groove (also referred to as “a weld line”), and an area sensor that receives a reflected light from the work pieces. This apparatus is configured so that it calculates a position of the welding groove of the work pieces using the two-dimensional position data based on the intensity signal of the reflected light. However, in the disclosure of the Japanese Patent No. 3228777, typically, end points of the projected laser slit are deformed due to influences of a gap width of the welding groove, a shape of the welding groove surface, a secondary reflection, etc. For this reason, the measured welding-groove position of the work pieces may be inaccurate if only the two-dimensional position data is used. For example, the deformation may be such that one of the end points of the projected slit laser may stretch into the welding groove gap, and the end point may be offset from the true welding-groove position, especially when the gap width of the welding groove is relatively large.
Moreover, an apparatus disclosed in Japanese Unexamined Patent Application No. 2000-346637, includes a displacement sensor that carries out a two-dimensional shape measurement of the welding groove of overlapped pipes to be welded, and a rotating device that is inserted down into the welding groove located in the pipes and rotates the displacement sensor. The displacement sensor is configured to measure a displacement of a welding groove surface of the pipes by scanning the surface with a spot-laser beam. This kind of apparatus is especially suitable to measure welding groove shapes of pipes. However, this apparatus has disadvantages in the tact time (cycle time) of welding especially when the tact time is thought to be important, even when the subject pipes are with the same shapes. That is, the apparatus requires a removal and attachment of the displacement sensor for every measurement.